The invention concerns a spectroscopic imaging method and system for exploring the surface of a sample.
It is particularly useful in Raman spectroscopy. It can equally be used in other forms of spectroscopy such as photoluminescence, fluorescence or cathodoluminescence.
Raman spectroscopy is an analytical technique providing molecule-specific information about a sample. When monochromatic light or radiation strikes a sample, the sample interacts with the light. A portion of the incident radiation may be scattered by the sample. Scattered radiation may contain both an elastic component, in which radiation frequencies remain unchanged, and inelastic components with altered frequencies. Elastically scattered components are called Rayleigh scattering. Inelastic components, if caused by light interacting with the vibrations of molecular bonds, are called Raman scattering.
A prior Raman analysis apparatus is described in a paper “Raman Microprobe and Microscope with Laser Excitation”, M Delhaye and P Dhamelincourt, Journal of Raman Spectroscopy, 3 (1975), 33-43 and the document FR 2 253 410. A sample is irradiated with monochromatic light from a laser, and the scattered light is passed through a monochromator in order to select a particular radiation of the resulting Raman spectrum. The monochromator comprises an entrance slit, onto which an optical system of the apparatus focuses an image of an illuminated point or line on the sample. A further optical system focuses an image of the entrance slit onto an exit slit. Between the entrance slit and the exit slit the monochromator has a dispersive device such as a diffraction grating, which has the effect of splitting the incoming Raman spectrum into a range of angles, depending upon frequency. The relative positioning of the exit slit and the diffraction grating thus selects the desired line of interest in the Raman spectrum.
Raman microscopy gained popularity during the last decade because of its capability to analyze microscopic samples down to the size of the sub-μm level. In a Raman microscope, the excitation beam is guided into and the signal beam from an objective lens that serves as focusing and collecting optics.
Another prior art Raman microscope is described in the document U.S. Pat. No. 7,102,746. It discloses a compact Raman spectrometer that may be assembled as an attachment onto infinity corrected light microscope. In order to reduce the size of the spectrometer, this one comprises a laser diode, small optics having small aperture and a CCD detector.
The intensity of a Raman band may be mapped over a two dimensional area or a three dimensional volume of sample by measuring a spectrum on each spot within the sample area or volume, thereby creating a two-dimensional or three dimensional Raman image of sample. Spectral images are useful for visualizing composition distribution on sample. Two dimensional spectral mapping can be done by moving either sample in the X-Y direction, or the laser spot using a pair of galvanometric mirrors with orthogonal scanning axes.
Nevertheless, with classical CCD detectors, the process for transferring the data from the CCD detector to the central unit is slow. One can obtain only few spectra per second even if the energies detected on the pixels have high intensity leading to short exposition times. Spectral data are sent spectrum per spectrum from the CCD detector to the central unit. It needs approximately two hours to obtain a Raman image having a definition of 50 points×50 points. Times required to obtain a Raman image with classical Raman microscope are too important.